1. Technical Field
The present disclosure is generally related to an apparatus and a method of prioritizing random access in a multi-user wireless communication system.
2. Description of the Related Art
IEEE (Institute of Electrical and Electronics Engineers) 802.11 Working group is currently in the process of standardizing the next generation WLAN (Wireless Local Area Network) technology under the 802.11ax Task Group. The primary goal of the Task Group is the improvement of spectrum efficiency to enhance the system throughput/area in high density scenarios of access points (also called as “APs”) and/or terminal apparatuses (also called as “STAs”). Among the various technologies being proposed, Orthogonal Frequency-Division Multiple Access (OFDMA) and uplink multi-user transmission are two key technologies that 802.11ax Task Group has adopted to achieve the throughput improvement goals.
In the past, 802.11 STAs operated on a 20 MHz channel bandwidth granularity, i.e., the smallest unit of channel bandwidth that a 802.11 STA could operate on was 20 MHz. But with the adoption of OFDMA, 802.11ax STAs, also known as High Efficiency (HE) STAs, can operate on sub-20 MHz channels. This means that several HE STAs can simultaneously transmit data on different sub-channels within the same 20 MHz channel. However, certain conditions have been imposed on such multi-user OFDMA transmissions, such as: all STAs taking part in a multi-user OFDMA transmission need to synchronize their transmissions to start at the same time point and to end at the same time point as well. Since the STAs are not aware of each other's transmission timings, this necessitates that the central controller, which is an Access Point (AP) in the case of WLANs, control the timings of the multi-user transmissions.
In 802.11ax, this is achieved by an AP that transmits a special control frame called a Trigger frame. The Trigger frame carries information such as the identity information of each of the STAs that may take part in the multi-user transmission, the transmission duration, the sub-channel (known as Resource Units, RU) allocation for each STA and other necessary information. STAs that are indicated in the Trigger frame transmit their respective frames on the allocated RU after a fixed interval of time (SIFS) since the end of the Trigger frame. This arrangement works fine when the AP has enough information regarding the STAs taking part in the multi-user transmission such as buffer status, STA operating state etc. But, there are cases where the AP may not have adequate information about the STAs to perform the necessary RU allocation in an efficient manner. In such cases, it is beneficial to allocate RUs to a group of STAs with similar characteristic and let the group of STAs contend for the RUs based on their actual needs. To meet such needs, the Uplink OFDMA Random Access procedure has been introduced in 802.11ax, hereon referred to as “UORA”. Details of UORA is found in IEEE802.11-15/1105r0, UL OFDMA-based Random Access Procedure and in section 9.58.2.5.1 of IEEE802.11-16/0024r0, Proposed TGax draft specification. See, for example, IEEE Std 802.11-2012.